Hi,
It does seem strange that the behaviour has changed.
If it worked in iChat I would have thought it would have worked in Messages.
As you may remember I did say that the Bonjour Account inniChat 6 (and earlier) could be set to Send As I Type and this seemed to cross over if you used a Yahoo account in iChat 6.x.x (Messages is coded as version 6.1 already meaning it is using the iChat "Engine" if you like).
AIM (for PCs at least) have for a long time also offere "Real Time" which is Send as I type stuff but is not compatible with iChat.
I have not seen anything that suggest that Messages is picking up on this feature.
Currently I have no real explanation.
Glad you have found a work around.
10:42 PM Friday; February 17, 2012
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.3)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously